CAUSE OF THE BLOOD*S MOTION IN THE VEINS. 93 



generally accompanied by two veins ; their aggregate 

 capacity is consequently greater, and the blood which 

 they contain bears the proportion of 9 to 5, to that in the 

 arteries. Their course is straighter and they more fre- 

 quently anastomose than the arteries. The larger veins 

 are generally enclosed in a common sheath with the ar- 

 tery that is going to the part whence the vein has come ; 

 the smaller veins are unaccompanied by arteries and are 

 more superficial in their situation. The blood brought 

 from all parts of the system, is collected, as we have be- 

 fore observed, into two great trunks which pour it into 

 the venous auricle of the heart. 



As the circulation has now been described, it may 

 naturally be divided into two parts, or stages, in reference 

 to its relation with the heart, viz. from the heart, through 

 the lungs, back to the heart ; then through every part of 

 the body, back to the heart again. The former portion of 

 the course is called the lesser, (*r pulmonary circulation ; 

 the latter, the greater, or systemic circulation. 



Emily. You do not mean certainly, that there are 

 two complete circles described by the course of the 

 blood ? 



Dr. B. No ; but that the course is rather represen- 

 ted by the arcs of two circles, one larger than 

 the other, connected by their extremities, in 

 this manner. Here you see that from which- 

 ever ventricle the blood is sent out, it does 

 not return to the same ventricle till it has 

 completed the whole circulation, both great- 

 er and lesser. 



Emily. I do not understand how the blood is moved 

 in the veins contrary to its own gravity, as it is in some 

 parts of the body, for you said that the action of the 

 heart ceased in the smaller arteries. 



Dr. B. This indeed is a very obscure point in phy- 

 siology, though the theories that have been made to ex- 

 plain it, have been neither few, nor lacking ingenuity. 

 Some have attributed it to the pressure of the atmos- 



